Quantum entanglement is a quantum mechanical phenomenon in which the quantum properties of two or more objects are described with reference to each other, even though the individual objects may be spatially separated. Two photons, A and B, are said to be entangled and form a bi-photon system when, if one property of the photon A is known, then the corresponding property of photon B is also known. For example, if two photons are prepared in an entangled state, and one of the photons is observed to have a specific polarization, then the other photon will have a known polarization.